In the winter of 2023, health experts across the European continent started noticing something weird. Cases of whooping cough, also known as pertussis, were rising. 

And it wasn't just happening in Europe. US health officials also began reporting a spike in whooping cough cases. And in the UK, case numbers had risen to their highest point in two decades. 

By March 2024, cases had spiked higher in Europe than in the past decade (the European Center for Disease Prevention and Control (ECDC) did not release figures earlier than 2011).

Some 32,000 cases were reported across Europe between January and March 2024. According to the ECDC, the yearly average of pertussis cases in Europe is around 38,000.

If the trend continues, whooping cough cases could increase some tenfold in 2024 compared to a typical year.

According to figures outlined in the ECDC's latest report on the situation, the majority of European cases occurred in infants — the population for which whooping cough can turn deadly. The second highest group of reported cases occurred in 10-14-year-olds. 

These figures need to be interpreted with caution, Paul Hunter, a professor of medicine at the University of East Anglia in the UK, told DW. Cases could be higher than reported, he said. Because babies are at such high risk when it comes to whooping cough, they are far more likely than other groups of the population to receive a diagnosis. So many older members of the population might also have contracted whooping cough that's gone undiagnosed.

On the other hand, Hunter said it's also possible there's a sort of contagion effect going on. Because doctors are becoming more aware of whooping cough, they may be more likely to give a diagnosis than in the past, he said, which could also distort the numbers. 

Vaccine uptake in pregnancy

In its initial stages, whooping cough presents like a cold. Patients may experience a runny nose, low-grade fever, sneezing and an occasional cough. After a few weeks with pertussis, however, coughing fits characterized by a high pitched "whoop" may start. The illness is most severe in children and infants, and mild cases may not come with the associated whoop. This stage can last up to 10 weeks. 

Experts say they don't know why cases are spiking. As with most infectious disease outbreaks, the culprit could be that a range of different factors comes together at once. 

Along with an overall decline in vaccine uptake in babies, it could be the result of the steady decline in pertussis vaccinations during pregnancy across Europe. This would help explain what's happening with the infants. When babies are born, they have no protection against whooping cough if their mothers weren't vaccinated during pregnancy. 

"​​You only start vaccinating children against whooping cough at about eight weeks," Hunter said. "And most of the most severe disease is actually often before that."

Pertussis vaccination rates in pregnancy vary broadly across the European continent, according to the May ECDC report. In Spain, around 88% of pregnant people were vaccinated against pertussis in 2023. In the Czech Republic, where the population has seen a sharp spike in pertussis cases, only 1.6% had received the shot that same year.

In the UK, uptake among pregnant people has declined over the course of the past decade, from around 70% in 2016 to around 60% in 2023. 

COVID's role

On top of that, the spike might be at least partially due to what health officials refer to as a drop in population-wide immunity since the COVID-19 pandemic. With the strict protocols adopted during the pandemic to ward off SARS-CoV-2 — mask-wearing, handwashing, less mixing in public places — flu and strep cases reached historic lows. 

Since the pandemic ended, cases have been rising again. But Hunter says that can't fully explain the dramatic spike in whooping cough. 

That's because whooping cough wasn't present in the population at high levels before the pandemic. It was there, but it was rare. Not like the flu. And flu cases have maybe doubled in the years since the pandemic, Hunter said — they haven't risen tenfold, as we're seeing with whooping cough. 

Pertussis vaccine

The third likely complicating factor could be the whooping cough vaccines themselves, experts say.

The first pertussis vaccine was introduced in the mid-20th century in developed countries like the US, Canada and parts of Europe. Although it was very effective, it was associated with negative side effects. The sharp drop in uptake that subsequently caused led to outbreaks in the 1970s and 80s.

During the late 1990s and early 2000s, countries began introducing a second- generation pertussis vaccine.

This new iteration was an acellular, rather than a whole-cell, shot. Although it did not lead to the side effects associated with the first vaccine, it was slightly less effective, conferring immunity for a shorter period of time. 

Where to go from here

The rise in cases presents difficult questions to doctors working on whooping cough, Andrew Preston, a professor and whooping cough expert at the University of Bath in the UK, told DW. 

Boosters may be an option to lower spread, he said, but "it's not entirely clear how often you can boost without losing effectiveness."

"Or are we happy with the situation whereby as long as we can stop babies from becoming really ill, and dying, is that good enough, and everyone else just has to cope with some chronic cough once in a while?"

There are new pertussis vaccines out there, Preston said, some which could transfer "far superior" immunity compared with the two vaccines available now. 

But he said these shots would be tricky to introduce into the current vaccine schedule. The pertussis vaccine is combined with five other vaccines in a single shot in the UK and most of Europe, so introducing a new one would require a restructuring of that combination vaccine. 

"You'd have to reformulate all those other vaccines, and that's just a monstrous task," said Preston.

Edited by: Derrick Williams